1. Field of the Invention
The present Invention relates to a video signal processing circuit having an inverse gamma correcting circuit for effecting an inverse gamma correction to three primary color input video signals or three primary color digital video signals, respectively.
2. Description of Related Art
Gamma correction is commonly effected to a video signal. When processing such a video signal by a signal processing circuit and displaying the processed signal on a display apparatus, it is general to use an inverse gamma correcting circuit for performing a correction opposite to the gamma correction provided in the signal processing circuit.
FIG. 1 shows the structure of a video signal processing circuit conventionally used for displaying video signals onto a plasma display panel (hereinafter, referred to as PDP). An analog luminance signal Y and first and second analog color difference signals (B-Y) and (R-Y) indicative of the video signals are supplied to a matrix circuit 1 by which analog three primary color video signals of red (R), green (G), and blue (B) are generated. The analog three primary color video signals RGB are supplied to analog/digital converters (hereinafter, referred to as A/D converters) 3r, 3g, and 3b and are converted to digital signals having a predetermined number of bits, respectively. The three primary color digital video signals are supplied to inverse gamma correcting circuits 4r, 4g, and 4b wherein the input signals are processed by an inverse gamma correction operation, thereby substantially linear three primary color video signals Rout, Gout, and Bout are produced. The three primary color video signals are then supplied to the PDP and an image corresponding to the video signals is displayed.
In the conventional structure described above, characteristics of the output signals are as shown in FIG. 2 with respect to the three primary color digital video signals RGB supplied to the inverse gamma correcting circuits 4r, 4g, and 4b. That is, the output signal characteristics with respect to the input three primary color digital video signals are in the form of characteristics of an exponential function or characteristics which are approximate to them.
In this conventional circuit, since input/output characteristics of the inverse gamma correcting circuits 4r, 4g, and 4b are of exponential function characteristics or characteristics approximate to them as described above, if a maximum dynamic range of the A/D converters 3r, 3g, and 3b to which the input signals of the inverse gamma correcting circuits are supplied is not completely used, the contrast of the image which is displayed on the PDP deteriorates in the manner of an exponential function. More specifically, when each of the A/D converters 3r, 3g, and 3b produces, for example, 8 bit data, the dynamic range will be a range from "0" to "255". If, however, the maximum signal level of the input video signals to be displayed, which are output from the A/D converter 3r, 3g, and 3b is, for example, at a level "Da" in FIG. 2, the dynamic range will be a range from "0" to "Da" which is smaller than 100% of the broadest range. In this way, a reduction of dynamic range occurs. Consequently, it becomes difficult to obtain desired contrast. In such a case, the luminance of the screen will be reduced to render the whole screen dark, so that it also makes difficult to obtain desired brightness characteristics.